1. Field of the Invention
The present invention relates to a device for eliminating in radar signals those corresponding to a radial velocity (also called "range rate") which is low in comparison with that of a large category of moving obstacles to be detected.
It is known to take advantage of the Doppler effect in radars to detect the moving obstacles that give rise to radar signals of low amplitude, amidst fixed obstacles corresponding to radar signals with high amplitude. Thus, in pulse radars of the coherent type and with a constant pulse repetition frequency Fr, the waves received after reflection on moving obstacles have a phase that changes from one pulse repetition period to the next, whereas the waves received from the fixed obstacles do not exhibit such phase shift changes. Due to this, the signals corresponding to moving obstacles have, after demodulation, complex components that vary sinusoidally with a frequency Fd called "Doppler shift" (or "Doppler frequency"), which is related to the radial velocity v and to the wavelength e of the radar by the formula Fd=2v/e.
Furthermore, the signals corresponding to fixed obstacles have a constant amplitude, and their spectrum consists of a series of discrete lines at the frequencies 0, Fr, 2Fr, . . . , nFr while the signals corresponding to moving obstacles is composed of discrete lines of the type mFr .+-.Fd.
It will be appreciated that it is possible to eliminate the signals corresponding to the fixed obstacles by using a fixed-echo elimination filter that blocks the signals with the frequencies 0, Fr, 2Fr, . . . , nFr.
It is also desirable to eliminate in certain radars such as the air traffic surveillance radars, the moving obstacles that have Doppler velocities which are low in comparison with the velocities of the moving obstacles of interest, for example clouds, or also the fluctuating fixed obstacles that have a certain Doppler velocity such as trees agitated by the wind. These various low-velocity spurious echoes are referred to as clutter.
2. Description of the Prior Art
Various systems have been proposed for eliminating the moving clutter at the same time as the fixed clutter. For example, one of them is based on the determination of the geographical situation of these clutters, and FIG. 1 shows an embodiment of such a system.
The radar signal S, with the real component I and imaginary component Q, is sampled in an analog-to-digital converter 1 and the amplitude of the samples is converted into a digital code before being applied to three parallel channels 2, 3 and 4 that lead to a first selecting device 5. Channel 2 corresponds to a direct link, i.e., without any signal processing and consequently without any loss of information. Channel 3 includes a fixed-echo elimination filter 6, that is a clutter-elimination filter, of conventional type, and channel 4 includes a device 7 that plots the map of the fixed clutter by determining, for example, the zone average level of the signal due to clutter. This map of the fixed clutter is used in the selecting device 5 to eliminate in the signals in channel 2 only those from the fixed-clutter zones marked by the circuit 7.
The output signals from the selecting device 5 are applied to three parallel channels 8, 9 and 10 that lead to a second selecting device 15. Channel 8 is a channel that includes only a threshold circuit 11 of the constant false alarm rate (CFAR) type. Channel 9 includes a moving-clutter rejection filter 12 followed by a CFAR threshold circuit 13, and channel 10 includes a circuit 14 for moving-clutter zone searching. When the presence of such zones is detected, the corresponding signal is used in the selecting device 15 to eliminate in the signals in channel 8 only those coming from the moving-clutter zones marked by the circuit 14.
The efficiency of the above-described system depends on the accuracy with which the fixed-clutter and moving-clutter zones are determined, i.e., on the number of these zones, which leads to use lots of hardware if it is desired to cover a maximum number of zones.
Another system for eliminating fixed and moving clutters is based on the measurement of the velocity and can be implemented as shown by the block diagram in FIG. 2. The radar signal S is applied to an analog-to-digital converter 21 similar to the converter 1 in FIG. 1, and the digital codes it delivers are applied to four parallel processing channels 22 to 25 whose first three channels 22, 23 and 24 lead to an OR circuit 27. The first channel 22 includes only a CFAR threshold circuit 26. The second channel 23 includes a fixed-echo elimination filter 30 followed by a CFAR threshold circuit 28, and the third channel 24 includes a moving-clutter elimination filter 31, also followed by a CFAR threshold circuit 29. The fourth channel 25 includes a moving-clutter velocity estimating circuit 32, this information being used to modify the transfer function of the filter 31 so as to match it to the detected velocity. With such a system with four channels in parallel, the signals from the OR circuit include neither those corresponding to the fixed echoes nor those corresponding to the moving clutter.
The performance of this system depends on the accuracy of the estimation of the mean velocity of the moving clutter, the size of the range window used by the moving-clutter mean velocity estimating circuit 32, and the variation of the instantaneous velocity of the moving clutter in this window. In addition, it may be necessary to use lots of hardware to implement such a system, which leads to high costs.
Furthermore, in both systems the use of a wobulation, i.e., of a known variation of the repetition frequency of the transmitted radar pulses restricts their performance, in particular for the estimation of the velocity of the moving clutter.